1. Field of Invention
This invention relates to paper making machinery, and more particularly to paper drying apparatus utilizing a single tier of rotatable cylindrical steam dryers extending from one end of the apparatus to the other.
2. Description of the Prior Art
It is well-known in the paper making art to use a raw stock dryer with a preliminary stage consisting of a single tier of steam dryers with a single felt support transporting the web or paper sheet from the paper press to a secondary drying stage. The felt improves contact between the web and the dryer, prevents wrinkling and folding of the paper, absorbs moisture from the web, admits the passage of heat through it, and gives support to the web through contact with the dryer.
A typical design of the individual dryers may be found in U.S. Pat. No. 4,476,637 issued to Justus et al.
The preliminary stage has the advantage of keeping the web, which is very weak as it emerges from the press due to its high initial water content, in constant contact with the felt thereby precluding any rips or tears in the web during this preliminary drying stage. However, because only one side of the web is exposed to the dryers in the preliminary stage, the secondary drying and subsequent stages were achieved through the use of an upper and lower pair of staggered rows of dryers. The web is threaded and sandwiched between a felt and a dryer from one of the rows in the second and subsequent stages, thereby drying one side of the web. The web would then be tensioned because it was unsupported as it moved up or down to be sandwiched between another felt and a dryer from the other row in such a manner as to dry the other side of the web. The space where the web is unsupported but tensioned as it traverses from one row to another is referred to as an "open" or "free" or "unsupported" draw. This alternation from one row of dryers to the other continued throughout the drying process. This general configuration may be found in U.S. Pat. No. 4,378,639 to Walker; U.S. Pat. Nos. 4,495,711 and 4,495,712 both issued to Justus; U.S. Pat. No. 3,778,908 issued to Notbohm; and U.S. Pat. No. 4,625,430 issued to Aula et al. The multi-tiered, staggered dryer configuration necessary to effect drying of opposite sides of the web was also dictated by space saving requirements.
This configuration of dual tiers of dryers, however, poses several disadvantages. The passing of the web unsupported by the felt from one tier to another, resulting in "open" or "free" draws, causes the web to be susceptible to folding, wrinkles or even tears. As the web is frequently unsupported by the felt, a machine directional draw or tension is required to run the web through the dryer, where centrifugal force tends to cause the web to move away from contact with the dryer surfaces. The "unsupported" web may also freely shrink in a cross dimension, thereby causing curl or "edge cockles". Similarly, start-up of this apparatus is difficult in that the web must be threaded through the dryers and various rollers. This is typically accomplished by the use of ropes along the edge of the web. This use of threading ropes not only presents a maintenance and safety concern but also reduces the width of the web which may be supported by the felt and results in instability of the edges of the web on the felt thereby reducing paper production yield and increasing distortions in the final product. Additionally, in order to prevent curl in the final paper product, a fine monitoring and adjustment of any temperature differential between the upper and lower row of dryers and felts must be maintained, as a temperature differential on each side of the web being dried may cause the web to curl. Furthermore, the double-tier design presents a very dense framework including steam pockets between the dryers, thereby not allowing the easy escape of moisture vapor, resulting in high humidity and inefficient cooling. Such a dense framework is difficult to access during maintenance or repair.
In accordance with the present invention, it has been found that by extending the single tier dryer configuration throughout the drying section of the apparatus from entrance to exit with a vacuum supported draw free transfer of the web from one dryer to the next, although retaining the drying of opposite sides of the web in discrete stages, all open draws are eliminated along with the many problems enumerated in the prior art two-tier dryer system, improving runnability. Use of this configuration throughout the dryer results in a lesser number of dryers being required to achieve drying of the web with resulting savings in energy and maintenance, than in the prior art system. The reduction in the number of dryers in such a single tier configuration is the result of three factors: the felt wrap is increased by decreasing the spacing between dryers, the ambient humidity is decreased because the enclosed pockets between the dryers are eliminated, the moisture vapor is free to escape and the sheet temperature is more readily maintained by minimizing the sheet length between dryers. Consequently, the drive load is reduced by having fewer dryers and less steamfit drag.